This new AREA grant proposal concerns indoleamine 2,3-dioxygenase (IDO), an enzyme induced in a variety of cell types by interferons and that catalyzes the oxidative cleavage of tryptophan. This enzyme is of interest because it is thought to participate in some anti-tumor and antimicrobial immune responses by limiting the availability of an essential amino acid. The correlation between IDO levels and antimicrobial activity are best developed in human-derived systems (and the investigator has played a significant role in this). In human fibroblasts and epithelial cells treated with IFN-gamma, Chlamydia psittaci and C. trachomatis growth was inhibited, and the effect could be at least partially reversed by exogenous tryptophan. A similar result was found for Toxoplasma gondii in monocyte-derived macrophages. There evidently are distinct pathways for IDO induction by type I and II interferons. Other factors such as IL-1, LPS, and C. psittaci infection itself can augment or induce IDO expression, consistent with IDO's role as a putative effector function subject to regulation by multiple inputs. In contrast, in mice, IDO is induced by LPS injection or during viral infection, but there has not been a demonstration of an in vitro correlation between IDO expression and antimicrobial response. C. trachomatis and R. prowazekii growth in mouse fibroblasts is inhibited by treatment with IFN-gamma, but IDO was not detected, and tryptophan had no effect. The investigator considers it possible that IDO induction in mice may require more complex conditions than in humans and may not be seen in vitro only upon treatment with IFN-gamma: co-stimulatory factors may be required. His hypothesis is that IDO is in fact positively correlated with resistance to infection to C. psittaci in mice, and he will test this hypothesis in the proposed studies. He has three Specific Aims: 1) He will make a murine cDNA library, 2) From this he will obtain a mouse IDO cDNA clone, and 3) he will test his hypothesis that IDO expression correlates with resistance to infection in C. psittaci-infected mice. In these tests, he will quantitate IDO mRNA and enzymatic activity from various tissues of mice given immunizing or lethal infections and upon secondary infection. He also will compare H-2 matched chlamydiae-resistant and susceptible mouse strains. This work will improve our understanding of IDO regulation and its differences in humans versus mice. The mouse IDO cDNA will be useful in future studies better characterizing a potentially important immune effector mechanism.